1. Field of the Invention
The invention relates to a hot-melt adhesive composition in the form of a granulate or powder, wherein at least some of the polymeric constituents contain moisture-reactive groups. The invention further relates to a process for producing such hot-melt adhesives and also to a process for bonding planar objects or moldings by using a reactive granulate or powder. Such reactive hot-melt adhesives which undergo secondary crosslinking under the influence of moisture may contain isocyanate end groups by way of moisture-reactive groups or they may contain silane groups.
2. Discussion of the Related Art
Reactive, one-component, moisture-curing polyurethane hot-melt adhesives are moisture-curing or moisture-crosslinking adhesives which are solid at room temperature and which are applied as an adhesive in the form of their melt and which have polymeric constituents that contain urethane groups as well as reactive isocyanate groups. In like manner, moisture-curing or moisture-crosslinking hot-melt adhesives may contain silane groups by way of reactive groups; in principle, such groups may have been produced on the basis of polyurethane prepolymers, but they may also have been produced from polyamides and other polymers with the aid of organofunctional silanes by means of reactions analogous to polymerization. As a result of the cooling of this melt after it has been applied and after the substrate parts to be bonded have been joined, first of all a rapid physical setting of the hot-melt adhesive takes place by virtue of the solidification thereof. This is directly followed by a chemical reaction of the isocyanate groups or silane groups that are still present with moisture from the environment so as to produce a crosslinked infusible adhesive. Reactive hot-melt adhesives based on isocyanate-terminated or silane-terminated polyurethane prepolymers are described, for example, by H. F. Huber and H. Müller in “Shaping Reactive Hotmelts Using LMW Copolyesters”, Adhesives Age, November 1987, pages 32 to 35.
Reactive polyurethane adhesives/sealants are distinguished by a very high performance profile. In recent years it has therefore been increasingly possible for new applications for these adhesives/sealants to be developed. Compositions for such adhesives and/or sealants are already known from a great many patent applications and other publications.
In addition to many advantages, these hot-melt-adhesive compositions also exhibit a number of disadvantages that are contingent upon the system. The moisture-curing hot-melt adhesives are very sensitive, depending on the system, to the ingress of moisture from the surrounding atmosphere. Therefore they are customarily packaged in moisture-proof casks, ordinarily being kept in these casks in the form of a compact block. This means that, at least in the case of large-volume application from barrels, barrel melt facilities are necessary for the application, so that the hot-melt adhesive has to be fused, subject to exclusion of moisture, and conveyed into the application unit. In respect of non-reactive hot-melt adhesives it has been known for a long time—and it is the state of the art—to offer them for sale not only in the form of compact blocks to be fused but also in the form of a free-flowing lumpy material, i.e. in the form of granulate, tablets, powder and similar particulate states. The advantages of such marketable forms consist in easy packaging and storage in bags, sacks or “BigBags”; moreover, such particulate hot-melt adhesives can be conveyed easily, be it by suction, pneumatic conveying or by charging into silos from which the particulate adhesive particles can fall directly into the fusing unit in free fall. It is therefore desirable also to be able to offer the moisture-reactive hot-melt adhesives for sale in pourable or free-flowing form.
Document DD 280 540 A1 describes a process for producing adhesive bonds by means of hot-melt adhesives. To this end, it is proposed that a first hot-melt adhesive preparation consisting of an EVA copolymer, a hydroxyl-functionalized and/or carboxyl-functionalized oligomer or high polymer and a second preparation consisting of an EVA copolymer, tackifying resins and a polyfunctional isocyanate in granulate form are premixed and then supplied to a fusing extruder in such a way that the dwell-time in the extruder at a temperature from 330 degrees K to 480 degrees K is maintained within the range from 10 s to 600 s and the mixing ratio of the two preparations amounts to 1:5 to 5:1 or both hot-melt-adhesive preparations are melted separately and mixed intensively shortly prior to application. One-component, moisture-curing hot-melt adhesives in granulate form are not disclosed in this document.
U.S. Pat. No. 4,384,083 describes hot-melt adhesives on the basis of granulated, polyether-based, thermoplastic polyurethanes and granulated polyaminoamides. In this patent it is described that the (isocyanate-terminated) polyurethane component and the polyaminoamide component are mixed in granulate form and then, while fusing, are firstly subjected to low shear and then to high shear. The extrudate is then either to be applied directly onto the substrate to be adhesion-bonded or it can be granulated into pellets and used later. The extrudate can moreover be prepared in the form of preformed film or in the form of powder, or it can also be dissolved in solvent. Pourable or free-flowing, one-component, moisture-curing hot-melt adhesives are not disclosed in this document either.
DE 195 41 923 describes a reactive system in granulate form on the basis of resin/polyurethane/EVA mixtures containing a reactive acrylate, epoxy resin or (blocked) isocyanate groups as well as hydroxy components. This system is intended to be capable of crosslinking by thermal means.
EP 832 953 describes a pulverulent reactive polyurethane composition on the basis of a blocked polyurethane prepolymer as well as non-functional urethane compounds based on monoisocyanates and monohydric or polyhydric alcohols or amines or alternatively polyisocyanates and monofunctional alcohols or amines. Textile bonded joints and also chill-casting processes (“slush molding”) are specified as a use for this hot-melt resin.
WO 93/25599 describes a process and mixtures of substances for producing reactive hot-melt compounds, which may be used, above all, as hot-melt adhesives, from meltable hydroxy-functional or amino-functional polymers or polymer mixtures which pass over into the liquid state at temperatures above 40 degrees C. and pulverulent, solid, surface-deactivated polyisocyanates. This document discloses on page 16 that carrier-free, reactive hot-melt compounds are obtained from the aforementioned compositions in the form of films, caterpillars, meshes, as granulate or as an unformed body by the liquid, uncrosslinked adhesive being applied onto a carrier which bears a non-adhering or adhesive surface and which is provided superficially with a release agent. After the adhesive has cooled, it can be released from the surface of the carrier and either stored temporarily or applied onto the substrate surface to be attached by adhesion. According to this document, reactive hot-melt powders are obtained by the liquid mixture being extruded onto a moving, adhesive coated belt conveyor and cooled there to below the melting point. After solidification, the hot-melt adhesive can be released from the conveyor belt and comminuted in accordance with known methods, e.g. by cold milling, so as to form granulate or powder and can optionally be sieved into the desired fractions. Moisture-curing hot-melt adhesives in granulate form, however, are not disclosed in this document.
In the paper by V. Neuenhaus entitled “Kapazitive Füllstandsüberwachung an Granulatbehältern”, Adhäsion, Kleben & Dichten, number 1-2, 2001, pp 26-28, it is explained that moisture-curing, one-component hot-melt adhesives cannot be granulated, on account of their strong reactivity with moisture.
In view of this state of the art, the inventors have set themselves the task of making available one-component, moisture-reactive hot-melt-adhesive compositions that are capable of being produced and used in free-flowing or pourable form.